I. Field of the Invention
The present invention relates to the field of adhesive and coating compositions and, more particularly, to a cationically charged adhesive or coating comprising a polymer which is dispersible or dissolvable in water. Because cationically charged adhesives or coatings can absorb to paper fibers, there is no deposition or substantial accumulation of sticky materials during a paper recycling process. Thus, the adhesives and/or coatings according to the present invention may be repulped and recycled along with paper during a paper recycling process without the problems associated with prior adhesives.
II. Description of the Prior Art
Conventional adhesives and coatings have long been a problem for the paper recycling industry because they tend to agglomerate and form so-called “stickies” that deposit on felts, press rolls, and drying cylinders during the paper recycling process. The stickies cause the machinery to run less efficiently and break down more often. The presence of stickies in the recycling process also reduces the quality of the recycled paper by, inter alia, forming holes and/or cosmetic flaws, such as dirty points, in the paper.
Technologies currently being evaluated by adhesive manufacturers in an effort to improve the paper recycling process can be divided into two categories: i) recoverable adhesives and ii) water soluble/dispersible adhesives. Recoverable adhesives can further be divided into two types: i) adhesives that have a high glass transition temperature or melting temperature, i.e., hot-melt adhesives and highly cross-linked adhesives, such that they will not soften and breakdown under repulping conditions; and ii) adhesives that have a density significantly lower or higher than the density of water and of fibers.
Recoverable adhesives unfortunately have several drawbacks. First, the adhesives with a high glass transition temperature or high cross-link degree usually exhibit substantially no spreading on the substrate at room temperature and, therefore, cannot be used as pressure sensitive adhesives (PSA's). Second, adhesives having a density lower or higher than that of fiber cannot be easily separated from the pulp furnish because those adhesives usually form large particles with the fibers and fillers during paper recycling and papermaking. Because the majority of the particles in the pulp furnish are fibers and fillers rather than adhesive particles, the final density of the aggregated particles is close to that of the fibers and fillers. Thus, the particles are difficult to remove. Third, only a limited number of high and low-density polymers exhibit adhesion-on-contact.
Adhesives and coatings that are soluble or dispersible in water have the advantage that they do not form stickies during the paper recycling process. Conventional compositions for water soluble/dispersible PSA's and coatings are described in various patents. For example, U.S. Pat. No. 5,928,783 discloses a PSA containing an aqueous emulsion polymer made by reacting at least one ethylenically unsaturated monomer with about 0.1 to 5% by weight, based on the total weight of the monomer, of a water soluble or dispersible polymerizable surfactant having a terminal allyl amine moiety. The ethylenically unsaturated monomer is defined to exclude ionic monomers and is preferably an alkyl acrylate monomer. An ionic monomer may optionally be used in addition to the ethylenically unsaturated monomer in an amount of from about 0.01 to about 10% by weight, based on the amount of ethylenically unsaturated monomer. Preferred ionic monomers are acrylamido methyl propane, sulfonic acid, styrene sulfonate, sodium vinyl sulfonate, acrylic acid, methacrylic acid, and the C4–C8 alkyl half esters of maleic acid, maleic anhydride, fumaric acid, and itaconic acid.
U.S. Pat. No. 5,574,117 relates to an acrylic polymer suitable for use as a PSA. The acrylic polymer, which contains no surfactant, may be prepared by bulk polymerization of a monomer having an acrylic acid-based monomer, a sulfur compound, and a polymerization initiator. The acrylic acid-based monomer unit has a number-average molecular weight (Mn) in the range of 1,000 to 1,000,000, a glass transition temperature of −80° C. or higher and a molecular weight distribution (Mw/Mn) of 5 or less. A PSA containing the acrylic polymer has the advantage of having a high heat resistance.
U.S. Pat. No. 5,714,543 pertains to a polyurethane PSA containing water soluble polymer additives having enhanced water uptake. Appropriate water soluble polymer additives have a solubility parameter ranging from 8 to 14 in solvents that exhibit poor hydrogen bonding, e.g., hexane. The polymer additives do not contain moieties which will react with isocyanates.
U.S. Pat. No. 5,380,779 sets forth a PSA composition that is water-dispersible when exposed to acidic conditions. The PSA composition comprises a terpolymer of a hydrophobic monomeric acrylic or methacrylic ester of a non-tertiary alcohol (monomer A); a polar monomer co-polymerizable with monomer A and having carboxylic, sulfonic acid or hydroxyl functionality or combinations thereof (monomer B); and a water soluble or water dispersible macromolecular monomer of the formula X—Y—Z, where X is a co-polymerizable moiety containing a double bond, Y is a divalent linking group, and Z is a water dispersible polymeric moiety. The macromolecular monomer is co-polymerizable with monomer A and monomer B. The combined acid groups of monomer B are neutralized from 5 to 100% based on the total amount of acid groups. The PSA composition further comprises tackifiers and/or plasticizers at a level such that the ratio of terpolymer to additives ranges from 0.2:1 to 5:1.
U.S. Pat. No. 5,439,748 discloses a water soluble or dispersible PSA composition that exhibits water solubility or dispersibility even in an acidic aqueous solution. The PSA comprises a sulfonate-containing copolymer obtained from 20 to 99% by weight of an alkoxyethyl acrylate, 1 to 30% by weight of a styrene sulfonic acid salt, and up to 50% by weight of a monomer mixture of acrylic acid-caprolactone adducts.
U.S. Pat. No. 5,489,642 describes a PSA which exhibits good adhesive properties even in thin layers and ensures satisfactory repulpability even in hard waters. The contact adhesive comprises a graft polymer in which 3–20% by weight of vinyl compounds having sulphonic acid salt groups are grafted onto a copolymer of (meth)acrylates and (meth)acrylic acid. The adhesive also includes water soluble plasticizers to render it tacky.
U.S. Pat. No. 5,169,441 pertains to charge reversal of finely divided fillers and pigments, such as clay, titanium dioxide, calcium carbonate, silicas, and silicoaluminates and discloses a paper coating composition. The fillers and pigments are treated with water soluble cationic polyelectrolyte polymers composed of the reaction product of epichlorohydrin and compounds containing 50 to 80% of cyclic quaternary functional groups.
Although conventional water soluble or dispersible adhesives and coatings do not necessarily form stickies that could damage paper recycling and papermaking machinery, the accumulation of dissolved or dispersed compounds is still an unsolved problem of paper recycling. It is theorized that the presence of accumulated adhesives or coatings in the paper making white water will affect other processing polymers used in papermaking, such as retention aids, sizing agents and strength additives. Further, accumulated adhesives in the white water will precipitate, if present at a sufficiently high concentration. Thus, it is desirable to form a recyclable adhesive or coating that would not require removal by a screen from the water used for paper recycling. Absent this generally performed screening step, the paper recycling process would be more efficient. It would also be advantageous to develop an adhesive or coating that may be recycled along with paper without concerns that the quality of the recycled paper might suffer.